By Jeff Mason and Aaron Maasho ADDIS ABABA (Reuters) - Barack Obama met the Ethiopian prime minister on Monday on the first visit by a serving U.S. president to a nation with one of the fastest-growing economies in Africa but which has often been criticized for its rights record. Talks with Prime Minister Hailemariam Desalegn are expected focus on security and the threat of the Islamist militant group al Shabaab in Somalia. Obama, who arrived from Kenya on Sunday, also wants to boost business ties with Africa.